Cleanup the shim code

[Manishearth]

• We now write directly to RETURN_PLACE instead of creating intermediates
• tuple_like_shim takes an iterator (used by [WIP] Make derive(Clone) memcpy on enum variants containing only Copy types #47867)
• tuple_like_shim no longer relies on it being the first thing to create blocks, and uses relative block indexing in a cleaner way (necessary for [WIP] Make derive(Clone) memcpy on enum variants containing only Copy types #47867)
• All the shim builders take dest, src arguments instead of hardcoding RETURN_PLACE

r? @eddyb

[Manishearth]

r? @nikomatsakis

This is a precursor to my patches for #47796 (which may need an rfc). @eddyb noticed that the tuple shim code was suboptimal and since I'm fixing that anyway might as well get these cleanups through.

[eddyb]

Can you name this dest?

[Manishearth]

done

[eddyb]

Isn't this also 5 lines above?

[Manishearth]

The confusion was "is it 0-indexed variant index, 1-indexed variant index, or the discriminant value". Basically, the fact that it says that it's zero for structs and unions suggests that it's perhaps nonzero for enums, and it's better if folks don't need to verify this.

[eddyb]

You can just edit that part instead of adding more elsewhere... also you wrote AdtDef::fields instead of variants. And this shouldn't be in this PR anyway.

[eddyb]

Actually structs and unions have exactly one variant so the rephrasing can be even more enum-focused.

[Manishearth]

fixed

[eddyb]

The newline seems misplaced. Also, we should avoid using "discriminant", because that's a different thing that may not be equal to the variant index. All the relevant code uses "variant index" (not "variant number").

[eddyb]

Please put this before rcvr_field and rename that to src.

[eddyb]

Please pass in ret_field from below as the destination (and rename it to dest_elem - also rcvr_field should be src_elem).

[eddyb]

This comment is unfinished.

[eddyb]

Please make this function also write in-place.

[eddyb]

Can you name this dest_field and put it before src_field?

[eddyb]

This should be named src not rcvr.

[Manishearth]

done

[eddyb]

This should be named previous_field, I think.

[eddyb]

dest.i instead of return.i

[eddyb]

cc @arielb1

[nikomatsakis]

Seems pretty good to me. A few nits/questions.

[nikomatsakis]

is there a test for whatever motivated this commit?

[Manishearth]

Without this commit everything ICEs. Everything.

Any code instantiating a clone shim of tuples or arrays or closures (libcore does) will break because the type used will be TySelf (not TyArray or TyClosure or whatever) and we'll get errors because we're trying to call .index() or .field() on a place that can't be indexed. I can leave a comment to this effect somewhere.

[nikomatsakis]

I mean I can see why this is wrong, but I'm a bit confused -- the code works now, afaik, right? Or is this just patching up some earlier commit of yours?

[eddyb]

It breaks once the change to the array shim is made - anything that depends on the type (e.g. indexing or enum discriminants) starts breaking because it'd see an unsubstituted Self.

[Manishearth]

Thsi commit should be earlier in the series I think.

We basically don't hit it because we write to temporary Places and then copy and that works well. I don't recall the exact details, but eddyb can explain.

[nikomatsakis]

ok good enough

[Manishearth]

Oh right yeah this doesn't break the tuple stuff but it does break the array stuff (and the enum stuff from my other PR)

[nikomatsakis]

Nit: can we use rustfmt-style formatting here?

[nikomatsakis]

Hmm, I think this is ok, but it's interesting that we can observe the mutations even on unwind etc. Do you know off hand @eddyb if this is already true? Seems like something that we will eventually want to be legal, though. Effectively you're giving in an &mut to the (uninitialized) RETURN_PLACE, so we can do what we want there.

[nikomatsakis]

But I feel like this has been an interesting question at times in the UCG discussions (when precisely can we reuse places etc).

[Manishearth]

done

[nikomatsakis]

@bors r+

[bors]

📌 Commit 8a8f91b has been approved by nikomatsakis

[bors]

⌛ Testing commit 8a8f91b with merge dd29d3d...

[bors]

☀️ Test successful - status-appveyor, status-travis
Approved by: nikomatsakis
Pushing dd29d3d to master...